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ISBT Science Series (2006) 1, 3–8 ORIGINAL PAPER 1ED-07-02 © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Structure and function of red cell surface antigens Blackwell Publishing Ltd G. Daniels Bristol Institute for Transfusion Sciences, National Blood Service, Bristol, UK Introduction Currently there are almost 300 red cell surface antigenic determinants or blood group specificities recognized by the International Society of Blood Transfusion; most of these belong to 1 of 29 blood group systems. Each system represents a single gene or cluster of 2 or 3 closely linked homologous genes, giving a total of 34 gene loci. Each of these genes has been cloned and sequenced, with the exception of one, P, and there is a candidate gene for this. Consequently, we have a tremendous amount of structural information about the red cell surface. Despite this, there is still much we do not know about the functions of these antigens; much of what we do know has been deduced from their structures. We know almost nothing about the functions of the blood group polymorphisms. The blood group systems are listed in Table 1, with information on the structures of their antigens and their possible functions. Almost all blood group systems have a null phenotype, in which no antigen of that system is expressed on the red cells. Null phenotypes are generally rare. They are usually only found when individuals with these phenotypes may make antibodies to the missing proteins following immunization by blood transfusion or pregnancy. Such antibodies are then identified in immunohaematology reference laboratories. Null phenotypes usually result from homozygosity for inactivating mutations or even a gene deletion, so the whole protein is absent from the red cells and from everywhere else in the body, at any stage of development. Yet in many cases individuals with these rare phenotypes are apparently healthy. This reflects the functional redundancy of many cell surface proteins. Often, when one protein is missing, another can perform the same function in its absence. However, this is not always the case and null phenotypes have been very informative about the functions of blood group antigens both on red cells and in other tissues. Red cell surface antigens are macromolecules anchored in the lipid bilayer of the red cell envelope. There are three main types of macromolecules expressing blood group activity: proteins, glycoproteins, and glycolipids. Most blood group antigens are glycoproteins, with the specificity determined primarily either by the oligosaccharide sequence (e.g. ABO) or by the amino acid sequence (e.g. MN, Kell, Duffy, Kidd, Diego). The Rh antigens are non-glycosylated proteins, though the presence of an associated glycoprotein is required for antigenic expression. ABO antigens are also expressed on the carbohydrate moiety of glycosphingolipids. In this article red cell surface antigens will be discussed in terms of their functions or potential functions, under the following five headings: transporters and channels; receptors and adhesion molecules; enzymes; structural proteins that link the lipid bilayer to the membrane skeleton; and those structures that contribute to the glycocalyx, or cell coat. Membrane transporters and channels Membrane transporters and channels facilitate the transport of biologically important molecules in or out of cells. They are typically polytopic: that is, they go in and out of the membrane several times, have both termini inside the cytoplasm, and they generally have an N-glycan on one of the extracellular loops (Fig. 1). The Diego antigen, band 3, the red cell anion exchanger The Diego antigen, known as band 3, is one of the most abundant red cell surface glycoproteins. It crosses the membrane 12 or 14 times, is glycosylated on the fourth extracellular loop, and has a long cytoplasmic N-terminal domain that interacts with the membrane skeleton (Fig. 1). One of the major functions of the blood is to transport respiratory gases. In addition to carrying oxygen to the tissues, carbon dioxide must be carried away from the tissues, to the lungs. Carbon dioxide in the blood is hydrated to bicarbonate by carbonic anhydrase II (CAII), which is only present in the red cell cytoplasm. Band 3 acts as an anion exchanger, an antiporter that permits bicarbonate ions to cross the membrane in exchange for chloride ions. The bicarbonate that accumulates in the red cells after conversion from carbon dioxide is rapidly transported out of the cell. This increases the amount of bicarbonate that can be transported in the plasma, which greatly increases the quantity of carbon dioxide that the blood can convey to the lungs. Band 3 is part of the Rh protein macrocomplex described below. 3 4 G. Daniels Table 1 The blood group systems System Gene(s) Structure Possible functions ABO MNS P Rh Lutheran Kell Lewis Duffy Kidd Diego Yt Xg Scianna Dombrock Colton LW Ch/Rg H Kx Gerbich Cromer Knops Indian Ok Raph JMH ABO GYPA, GYPB P RHD, RHCE LU KEL FUT3 DARC SLC14A1 SLC4A1 ACHE XG, CD99 ERMAP ART4 AQP1 ICAM4 C4A, C4B FUT1 XK GYPC CD55 CR1 CD44 BSG CD151 SEMA7A GCNT2 B3GALNT1 AQP3 Carbohydrate Sialoglycoproteins Carbohydrate Polytopic proteins IgSF glycoprotein Enzyme Carbohydrate G protein-coupled superfamily Solute carrier Solute carrier Enzyme Sialoglycoproteins IgSF glycoprotein Enzyme Aquaporin IgSF glycoprotein Complement protein Carbohydrate Polytopic protein Sialoglycoproteins Complement control protein Complement control protein Glycoprotein IgSF glycoprotein Tetraspanin Semaphorin Carbohydrate Carbohydrate Aquaporin Glycocalyx Glycocalyx Unknown NH3/NH4+ or CO2 gas channel/transporter Adhesion/receptor – binds laminin Endopeptidase – processes endothelin 3 Unknown Chemokine receptor Urea transporter Anion exchanger, anchor to membrane skeleton Acetylcholinesterase Receptors Adhesion/receptor ADP-ribosyltransferase Water channel Adhesion/receptor – binds integrins C4 component of complement Glycocalyx Neurotransmitter transporter Anchor to membrane skeleton Complement inactivation Complement inactivation Adhesion/receptor – binds hyaluronan Adhesion/receptor Complexes with integrins in the membrane Adhesion/receptor Glycocalyx Unknown Water/glycerol channel Globoside Gill Band 3 in red cells has other functions: anchoring the plasma membrane to the membrane skeleton; acting as a binding site for haemoglobin, CAII, and glycolytic enzymes; and playing a role in red cell senescence. Kidd antigen: urea transporter The Kidd antigen spans the membrane 10 times and is glycosylated on the third extracellular loop (Fig. 1). It is a urea transporter: it transports urea rapidly into or out of the cell. When red cells enter the renal medulla, where there is a high concentration of urea in the blood, urea is transported rapidly into those cells. This prevents the rapid loss of water in this hypertonic environment and shrinkage of the cells. As the cells leave the renal medulla, urea is transported rapidly out of the cells, which prevents the entry of water and excessive swelling of the cells and defends against the cells from taking urea away from the kidney, which would reduce the urea concentrating capability of the kidney. Colton antigen: aquaporin 1 (AQP1) a water channel The Colton antigen is a member of the Aquaporin family of membrane channels. Aquaporins have six membrane-spanning domains (Fig. 1), but also have two cytoplasmic loops that contain Ala-Pro-Asn motifs. Tetramers of AQP1 in the red cell membrane form channels that enhance osmotically driven water transport. GIL antigen: aquaporin 3 (AQP3) a glycerol channel The GIL antigen is aquaporin 3, a glycerol and water channel. The Rh protein macrocomplex or Rh metabolon The proteins expressing the D and CE antigens of the Rh system cross the membrane 12 times but, unlike transporters, © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Ltd. ISBT Science Series (2006) 1, 3–8 Red cell surface antigens 5 Fig. 1 Diagrammatic representation of the proposed configuration of some polytopic proteins and glycoproteins with blood group activity. N = N-glycan. they are not glycosylated (Fig. 1). They are closely associated in the membrane with a glycoprotein, called the Rh-associated glycoprotein (RhAG), that has a similar structure to the Rh proteins, but is glycosylated and does not carry Rh antigen activity (Fig. 1). The Rh proteins are encoded by genes on chromosome 1, whereas the RhAG is encoded by a gene on chromosome 6. The Rh protein complex has generally been considered to be a tetrameric structure, with two molecules of RhD and/or RhCE and two molecules of RhAG, but recent structural analyses has suggested that a trimeric structure is most likely. The Rh genes, RHD and RHCE, have about 30% sequence identity to RHAG. Two more genes of the human Rh gene family, RHCG and RHBG, both with about 50% sequence identity to RHAG, are nonerythroid, but are expressed in kidney, liver, skin, and testis. RhD, RhCE, RhAG, RhBG, and RhCG are part of a large family of proteins found in archaea, bacteria, plants, and animals, known as the ammonium transporter/ methylamine permease/Rh (Amt/Mep/Rh) family. RhAG has about 25% homology with ammonium transporters in lower animal and plants, called the Mep proteins. Yeast cells require ammonium for growth and yeast cells lacking Mep proteins fail to grow in low levels of ammonium. The growth defect in mep-deletion yeast cells could be repaired by transfection of the cells with the human RHAG gene. Furthermore, Xenopus oocytes transfected with the RHAG gene took up methylamine, an analogue of ammonium, which untransfected cells did not. Rhnull cells, which lack Rh proteins and RhAG, have reduced rates of ammonium transport through the mem- brane. These results suggest that RhAG has the capability to mediate ammonium transport, but whether this is in the pro+ tonated ( NH4 ) or unprotonated (NH3) form is controversial. It has been suggested that RhAG promotes retention of ammonium in red cells in order to maintain a low level of + total blood ammonia ( NH4 + NH3), and possibly for transport of ammonium to the liver or kidney and subsequent removal from the body, thus protecting against ammonia toxicity in the brain. However, no direct evidence exists that the Rh complex functions to facilitate transfer of ammonium in or out of the red cells in vivo. In contrast, there is also some evidence that the Rh protein complex might function as a gas channel for CO2. The photosynthetic green alga Chlamydomonas reinhardtii depends on CO2 for photosynthesis. It has a protein homologous to the Rh family of proteins, encoded by a gene called RH1. When this alga is cultured in air, with low levels of CO2, there is high expression of RH1, but when it is cultured in high levels of CO2, there is low expression of RH1. When it is moved from low to high levels of CO2, there is substantial reduction in RH1 expression, but when moved from high to low levels of CO2, there is a substantial increase in RH1 expression. This suggests that the Rh1 protein might function as a CO2 channel in this microbe. The complex of Rh proteins and RhAG are part of a macrocomplex or metabolon of proteins in the red cell membrane, which also contains band 3, LW (ICAM-4), CD47, and glycophorins A and B, with band 3 at its core. It has been proposed that the macrocomplex involving Rh and band 3 functions as an oxygen/carbon dioxide gas exchange channel. The presence of such a gas exchange channel might be expected, as the primary purposes of the red cell are transport of oxygen and conversion of carbon dioxide to bicarbonate. The macrocomplex is ideally located to channel CO2 to and from carbonic anhydrase and oxygen to and from haemoglobin, as band 3 is anchored to both CAII and haemoglobin. There is no doubt that ancestral Rh proteins function as ammonium transporters. It is reasonable to speculate that, as a result of its evolutionary origins, RhAG is able to mediate transport of ammonium, and that its nonerythroid homologues RhBG and RhCG may still serve this function. However, the likely primary function of RhAG in red cells is to provide a gas channel for CO2, which like NH3 is a readily hydrated gas, and possibly for O2. It is unlikely that the more rapidly evolving Rh proteins have any direct transport or channel-forming functions, and may have a role in maintaining the correct shape of the red cells. XK protein The XK protein crosses the membrane 10 times (Fig. 1). It is not glycosylated, but is linked through a single disulphide bond to the Kell glycoprotein. The function of XK is not © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Ltd. ISBT Science Series (2006) 1, 3– 8 6 G. Daniels The IgSF is a very large family of adhesion molecules and receptors with different numbers of repeating immunoglobulinlike domains. ICAM-4 can bind LFA-1 (α1β2), α4β1, and αV integrins. Integrins are adhesion molecules present on haemopoietic and non-haemopoietic cells, but not mature red cells. They have a heterodimeric structure, consisting of different types of α and β chains, both of which cross the cell membrane once. αV subunit forms heterodimers with a variety of β subunits. Erythroblasts express both ICAM-4 (LW) and α4β1 integrin, whereas macrophages express αV integrins. Erythroblastic islands in the bone marrow comprise a cluster of erythroblasts around a macrophage, which is important for enucleation of the erythroblasts. Erythroblastic islands involve cell adhesion events and cytokine interactions that are critical in the regulation of erythropoiesis and apoptosis. It is possible that ICAM-4 on erythroblasts can bind integrins on macrophages and on other erythroblasts in the bone marrow and help to stabilize the erythroblastic islands. Furthermore, downregulation of α4β1 integrin on enucleated erythroblasts may assist in their release from the erythroblastic islands. ICAM-4 is part of the Rh glycoprotein macrocomplex and it has been speculated that, together with another adhesion IgSF molecule, CD47, it might assist in maintaining close contact between the red cell membrane and the endothelium of the capillary wall. It is probable that both Lutheran and ICAM-4 (LW) have a pathophysiological role in sickle cell disease by contributing to red cell adhesion to endothelial cells and development of vaso-occlusion. Lutheran Scianna The Lutheran glycoproteins, which have five immunoglobulin superfamily domains (V-V-C2-C2-C2), bind laminin, a glycoprotein of the extracellular matrix. Laminin consists of three chains, α, β, and γ, and exists in at least 12 different isoforms, derived from combinations of different α, β, and γ chains. The Lutheran glycoproteins bind specifically and with high affinity to isoforms of laminin that contain α-5 chains, called laminin 10 and 11. During in vitro erythropoiesis, Lutheran is the last erythroid surface marker to appear, probably at the orthochromatic erythroblast stage. For mature erythroblasts to leave the bone marrow they must cross the sinusoidal endothelium. Bone marrow sinusoidal endothelium has laminin isotypes 10 and 11 on its surface and this has led to speculation that the Lutheran glycoproteins are involved in the trafficking of mature erythroid cells from the erythroblastic islands of the bone marrow, across the sinusoidal endothelium, and to the peripheral blood. The Scianna antigen has one IgSF domain (V) and is erythrocyte membrane-associated protein (ERMAP). LW, intercellular adhesion molecule-4 (ICAM-4) Complement control proteins are cell surface glycoproteins that regulate the complement cascade. Glycoproteins of the complement control protein superfamily have between two and 30 repeating domains, which are characteristic of this superfamily. known, but XK shares some sequence homology with a family of neurotransmitter transporters. Absence of XK results in late onset muscular, neurological, and psychiatric symptoms, together with acanthocytic red cells. Receptors and adhesion molecules Duffy glycoprotein, the Duffy antigen receptor for chemokines (DARC) Like the transporters, the Duffy glycoprotein is polytopic; unlike the transporters, the amino-terminus is outside the membrane. This structure is characteristic of a very large superfamily of receptors, called the G protein-coupled superfamily, that bind many different ligands. The Duffy glycoprotein, also known as DARC, acts as a receptor for a variety of pro-inflammatory cytokines of both C-X-C and C-C types, including IL-8, MGSA, MCP-1, and RANTES. Duffy is present in many organs, and is abundant on postcapillary venules. On red cells it could act as a clearance receptor for inflammatory mediators. Blood group antigens belonging to the immunoglobulin superfamily (IgSF) ICAM-4 has two IgSF domains (C2-C2) and is part of the Rh glycoprotein macrocomplex. During erythropoiesis ICAM-4 first appears on erythroid cells on late proerythroblasts or early erythroblasts, about the same time as the Rh antigens. Ok The Ok antigen has two IgSF domains (C2-V) and is basigin (CD147). The Indian antigen, CD44 The hyaloadherin CD44 is a member of the Link Module superfamily of proteoglycans that contain a Link module, a structural domain of about 100 amino acids that is involved in ligand binding. CD44 is the predominant cell surface receptor for the extracellular matrix (ECM) glycosaminoglycan hyaluronan. The functions of CD44 are diverse, although the function of CD44 on RBCs remains unknown. Complement control proteins © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Ltd. ISBT Science Series (2006) 1, 3–8 Red cell surface antigens 7 Cromer The Cromer antigen is CD55 or decay-accelerating factor (DAF), which has four complement control protein domains and is attached to the membrane by a glycosylphosphatidylinositol (GPI) anchor. CD55 helps to protect the red cells from lysis by autologous complement by inhibiting the action of C3-convertases. However, the glycoprotein that is most important in the function of protecting the cell from autologous complement is another GPI-linked glycoprotein, CD59, which is not polymorphic and does not have blood group activity. CD59 prevents assembly of the membrane-attack complex. Dombrock The sequence of the Dombrock protein suggests that it belongs to a family of ADP-ribosyltransferases that transfer ADP-ribose to various protein acceptors. It is not known whether this structure is enzymatically active on red cells. Kell The Kell glycoprotein is an active endopeptidase and can process the vasoconstrictor endothelin-3 in vitro, but it is not known whether the Kell glycoprotein serves this function in vivo. Knops CR1 or CD35, the Knops blood group antigen, is a very large glycoprotein with about 30 repeating complement control protein domains. The major function of red cell CR1 is to bind and process immune complexes and transport them to the liver and spleen for removal from the circulation. Xg, CD99, and JMH The structures of the Xg and CD99, and JMH (CD108 or Sema7a) molecules suggest they might also have receptor or adhesion functions. MER2 The MER2 blood group antigen, CD151, belongs to the tetraspanin superfamily. Tetraspanins span the cell membrane four times, have short, internal N- and C-termini and one small and one large extracellular loop (Fig. 1). Although CD151 is probably not an adhesion or receptor molecule, it associates closely with certain integrins in cell membranes to generate complexes that bind laminin in basement membranes and are important in maintaining the integrity of the basement membrane. The very rare MER2-null phenotype results in disruption of basement membranes in the kidney, causing hereditary nephritis (all the patients required dialysis and kidney transplants), in the skin, causing epidermolysis bullosa, a severe blistering, and in the inner ear, leading to neurosensory deafness. The function of CD151 in the red cell membrane remains unknown. Red cell surface antigens with putative enzymatic activity Some red cell surface glycoproteins appear to be enzymes. Yt Yt is acetylcholinesterase, an enzyme that plays an essential role in neurotransmission. This structure is enzymatically active on red cells, but its function on red cells is not known. Structural proteins There are some membrane proteins that have a structural function to link the membrane to its skeleton. The membrane skeleton is a network of glycoproteins, mostly spectrin and actin, beneath the plasma membrane, which is responsible for maintenance of the shape of the red cell. Effective attachment of the plasma membrane to its skeleton is very important in maintaining the shape and flexibility of the red cell as it squeezes through the smallest capillaries. Band 3, the Diego glycoprotein, has an extended N-terminal domain attached to the membrane skeleton through ankyrin and proteins 4·2 and 4·1R. Individuals heterozygous for mutations with the gene for band 3 often have hereditary elliptocytosis or Southeast Asian ovalocytosis. Glycophorins C and D, the Gerbich glycoproteins, have a long C-terminal domains attached to the membrane skeleton through protein 4·1R and p55. A proportion of Gerbich-null red cells are spherocytic. In addition there is evidence that RhAG interacts with ankyrin, the Lutheran and XK proteins with spectrin, and CD44 with protein 4·1. Red cells lacking RhAG, XK, or, to a lesser degree, Lutheran glycoproteins and CD44 [In(Lu) phenotype] have some degree of abnormal morphology. Carbohydrate antigens The ABO and H antigens are carbohydrate histo-blood group antigens present on many tissues. On red cells most ABO and H antigens are on the N-glycans of band 3 and the glucose transporter (GLUT1), as well as being on minor glycoproteins and on glycolipids. The extracellular domains of glycophorins, that express the MNS antigens and are abundant on the red cell surface, are also heavily glycosylated. The functions of these carbohydrates on red cells are not known, apart from contributing to the glycocalyx or cell coat, an extracellular matrix of carbohydrate that surrounds the cell and protects it from mechanical damage and microbial attack. © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Ltd. ISBT Science Series (2006) 1, 3– 8 8 G. Daniels Conclusion The red cell surface is highly complex and the red cell membrane proteins serve a variety of functions: some designed to make the red cell an efficient transporter of respiratory gases, others to provide the red cell with additional functions, such as the removal of excess cytokines and immune complexes from the peripheral blood, and probably many others that we do not know about. Some structures on the red cell may be vestigial, having served their function during erythropoiesis or even at an earlier evolutionary stage of the organism. In addition, we know almost nothing about the biological importance of the polymorphisms that make the red cell surface proteins alloantigenic. Bibliography 1 Daniels G: Human Blood Groups, 2nd edn. Oxford, Blackwell Science, 2002 2 Daniels G: Functional aspects of red cell antigens. Blood Rev 1999; 13:14–35 3 Denomme GA: The structure and function of the molecules that carry human red blood cell and platelet antigens. Transfus Med Rev 2004; 18:203–231 4 Reid ME, Mohandas N: Red blood cell blood group antigens: structure and function. Semin Hematol 2004; 41:93–117 5 Storry JR: Review: the function of blood group-specific RBC membrane components. Immunohematology 2004; 20:206–216 6 Tanner MJA: Band 3 anion exchanger and its involvement in erythrocyte and kidney disorders. Curr Opin Hematol 2002; 9:133–139 7 Wintour EM: Water channels and urea transporters. Clin Exp Pharmacol Physiol 1997; 24:1–9 8 Proceeding of the International Conference on the Rh protein superfamily. Transfus Clin Biol 2006; 13:1–178 9 Westhoff CM: The Rh blood group system in review: a new face for the next decade. Transfusion 2004; 44:1663–1673 10 Bruce LJ, Beckmann R, Ribeiro ML, Peters LL, Chasis JA, Delaunay J, Mohandas N, Anstee DJ, Tanner MJA: A band 3based macrocomplex of integral and peripheral proteins in the RBC membrane. Blood 2003; 101:4180 –4188 11 Hadley TJ, Peiper SC: From malaria to chemokine receptor: the emerging physiologic role of the Duffy blood group antigen. Blood 1997; 89:3077–3091 12 Parsons SF, Spring FA, Chasis JA, Anstee DJ: Erythroid cell adhesion molecules Lutheran and LW in health and disease. Baillière’s Clin Haemat 1999; 12:729–745 13 Eyler CE, Telen MJ: The Lutheran glycoprotein: a multifunctional adhesion receptor. Transfusion 2006; 46:668– 677 14 Jentoft N: Why are proteins O-glycosylated? Trends Biochem Sci 1990; 15:291–294 © 2006 The Author. Journal compilation © 2006 Blackwell Publishing Ltd. ISBT Science Series (2006) 1, 3–8